Many diseases of the eye result in localized retinal disfunction. An example of this is glaucoma in which high intraocular pressure causes damage to the nerve fiber layer of the retaina. The areas of retinal disfunction are perceived subjectively by the patient as areas of decreased reaction to light or stimulation by light as is said in the art. If the damage is such that no light, however intense, will stimulate a particular area of the retina, i.e., generate electrical activity, then that area is said to be an absolute scotoma in the patient's visual field. If the retina can be stimulated, but only by light of an intensity higher than some average predetermined baseline, then the scotoma is said to be relative.
Present methods of measuring one's visual field are purely subjective. The dimensions of one's visual field are measured in terms of visual angle, and typically measure 90.degree. temporally and 60.degree. nasally and vertically from a point of fixation. The standard method of measuring a patient's visual field is called Goldman perimetry. In this method one eye of the patient is patched. The eye to be tested fixates on a target spot in the middle of a hemispherical globe which has a certain constant background illumination. Visual stimuli (i.e., light) of various intensities and areas are presented in standard ways at different locations on the surface of the hemispherical globe. With the presentation of each stimulus, the patient is asked to respond by a hand signal or other means as to whether he saw the stimulus while focusing on the fixed target spot. The responses of the patient are used as measurements to determine the visual threshold at various points in the patient's visual field, thereby delineating the presence of scotomas.
An example of the utility of the Goldman perimetry technique is in the diagnosis and monitoring of glaucoma. The diagnosis of glaucoma is based on the presence of scotomas which are characteristic of the disease as well as the appearance of the optic nerve and the intraocular pressure. The visual fields of glaucoma patients are monitored closely first as an aid to making the diagnosis, and secondly to detect any progression of these visual field defects or scotomas. Any progression of the scotomas implies inadequate control of the intraocular pressure and indicates the need for additional therapy.
A problem with this technique is that it is subjective and hence subject to the judgement of the patient. Thus any inattention due, for example, to fatigue, inability to understand the test, and age degrade the quality and reliability of the test. This technique can be automated, but is still subjective and hence subject to the same vagaries.
Other techniques for testing a person's visual field have included electroretinograms (ERG's) which measure the electrical response of the entire retina to light incident upon any part of one's visual field. In order to determine the health of particular segments of the retina from the ERG responses, each spot of one's visual field must be illuminated with a focused spot of light that is flashed many times over a period of about twenty seconds. To complete such a test takes a substantial amount of time. Consequently such techniques have not shown any advantage over the subjective or other methods of testing one's visual field.